This invention relates generally to radio frequency networks and more particularly to multi-port radio frequency networks.
As is known in the art, multi-port radio frequency networks have a wide range of applications. In one such application, the network is used as a power divider/combiner for distributing radio frequency energy between a first port of the network and a plurality of second ports of the network. In an array antenna application of such power divider/combiner, an array of antenna elements is coupled to the plurality of second ports. Energy fed to the first port during transmission is coupled to the array of antenna elements and, reciprocally, energy received by the array antenna elements is combined at the first port. One such array antenna is a phased array antenna wherein a plurality of electrically controlled phase shifters is coupled between the plurality of second ports of the divider/combiner and the array antenna elements. Energy fed to, or combined at, the first port of the power divider/combiner is collimated into a beam, such beam being directed by the phase shift provided by the phase shifters, in response to electronic signals fed to the phase shifters. In another array antenna, a radio frequency lens is used as the power divider/combiner, such radio frequency lens having a plurality of first ports, each being associated with a corresponding one of a plurality of simultaneously produced, differently directed collimated beams of radio frequency energy. Each one of such beams is formed by a common aperture provided by an array of antenna elements coupled to a plurality of second ports of the lens. In either the phased array antenna or the lens array antenna, it is generally desired that the plurality of second ports have a relatively high degree of electrical isolation between each one thereof and, in the case of the lens array antenna, it is also generally desirable that the plurality of first ports also have a relatively high degree of electrical isolation between each one thereof. This isolation is desired to reduce the effect of reflections generated in one of the "isolated" ports from adversely effecting another one of the "isolated" ports. For example, in the phased array antenna, it is desirable that any energy reflected by one of the phase shifters not couple into another one of the phase shifters. In the lens array antenna, when such is configured to transmit a beam of radio frequency energy, an amplifier, such as a travelling wave tube amplifier, is generally coupled between each second port, and the antenna element coupled to such second port, and thus, if one of the amplifiers is defective, such may reflect energy back into the lens and such energy will then subsequently couple into an adjacent second port, thereby degrading performance of the antenna. Further, when the lens array is configured as a receiving array antenna, a radio frequency energy receiver is generally coupled to each one of the plurality of first ports of the lens. Energy received by the array of antenna elements is directed, or "focussed", to a receiver coupled to one of the first ports in accordance with the angle of arrival of such energy. However, some portion of the energy "focussed" to the receiver may be also reflected by the receiver. In the absence of a high degree of electrical isolation between the first ports, such reflected energy may couple into another receiver coupled to an adjacent one of the first ports thereby adversely affecting the performance of the antenna system.
In each of the above array antenna applications, the required electrical isolation has generally been provided by a single power divider/combiner component having the requisite port isolation. More particularly, in the phased array antenna application, one type of power/divider component having a relatively high degree of electrical isolation between output ports is a matched corporate feed such as that described in FIG. 38a, and Pages 11-52 to 11-53 of a book entitled Radar Handbook, Merrill I. Skolnik, Editor-In-Chief, published by McGraw Hill Book Company, New York, New York (1970). As described therein, the feed frequently includes a plurality of matched two-way dividers in which the "out-of-phase" components of mismatched reflections are absorbed in terminating loads. While such network provides the desired electrical isolation between the output ports thereof, when constructed as an integral corporate structure the terminating loads are disposed within the structure thereby increasing the fabrication complexity and hence, fabrication cost. Further, the two-way dividers are arranged in cascaded rows, the number of two-way dividers in the rows increasing binarily from row to row. Thus, if, for example, the feed is to feed sixteen antenna elements, four rows of dividers would be required and power fed from the input divider to each one of the sixteen antenna elements must pass through four serially, cascade coupled, dividers. Since energy passing into a divider experiences some loss, it follows that power losses in the feed increase directly with the number of antenna elements in the array.